Signaling system



SIGNALING SYSTEM 6 Sheets-Sheet 1 Filed June 27, 1944 5 H h CMn A l U A h MST dHu7 um E u s. a R v. L B lll l) W- llll lhJ U N IIIIIYIJUI lllll lJ l 5 FE G5 A TTORNEV 5:. M. STAPLES ET AL 2,388,347

SIGNALING SYSTEM Filed June 2'7, 1944 6 Sheets-Sheet '2 Nov. 6, 1 945.

li-LM/CHAEL INVENTORS ELI-STAPLES ERJWYLOR By w 0 C imea ATTORNEY :Nov, 6, 1945. E. M. STAPLES ETAL 2,388,347

- SIGNALING SYSTEM Filed June 27, 1944 e Sheets-Sheet s POLARIZED 5;; POLAR/ZED h'.J.M/CHAEL INVENTORS EMSTAPLES Q 'ERJZYLOR Arron/gr Nov. 6, 1945.

FIG. 4

TLCZ

E. M.- STAPLES ETAL SIGNALING SYSTEM Filed June 27, 1944 IN I/E N TORS 6 She efcs-Sheet 4 H. JLMICI /AEL EM. STAPLES EZR. TAYLOR ATTORNEY Nov 6, 1945. l M. STAPLES ETAL SIGNALING SYSTEM I Filed June 27, 1944 6 Sheets-She et 5 a a T H.J.MICHAEL INVENTORS E. MSTAPLES BY E. R. 7A YLOR ATTORNEY Nov. 6, 1945. E. M. STAPLES ETAL SIGNALING SYSTEM Filed June 27, 1944 6 Sheets-Sheet 6 INVENTORS EMSTAPLES ER. TAYLOR BY ATTORNEY Patented Nov. 6, 1945 OFFICE SIGNALING SYSTEM Elliot M. Staples, Hohokus, N. J and Edmund H- Taylor, Pelham Manor, and Henry J. Michael, Astoria, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 1 Application June 27, 1944, Serial No. 542,356

11 Claims.

This invention relates to signaling systems and particularly to telephone systems in which voice frequency signaling currents are employed.

Objects of the invention are the provision of more reliable signal transmitting and receiving means in systems employin voice frequency signaling and the prevention of false operation in response to voice currents or other interfering currents.

This invention is a signaling system comprising means for transmitting signals consisting of an impulse of alternating current of a first frequency followed by an impulse of alternating current of a second frequency or for transmitting signals consisting of an impulse of current of the second frequency followed by an impulse of current of the first frequency and means for responding to such signals without responding to voice currents or other interfering currents. Similar transmitting and receiving means are connected to both ends of a toll telephone line, different frequencies being used for signaling in each direction thereover. The signal transmitting means is arranged to convert direct current signals to voice frequency signals of the desired character and the signal receiving means is arranged to convert the incoming voice frequency signals to direct current signals. Means are provided for opening the talking connection over the associated toll telephone line whenever signals are being transmitted or received thereover, the connection opening means being non-responsive to voice currents or other interfering currents having a signaling frequency component. Signal delay networks are provided both in the signal transmitting and in the signal receiving means for delaying the transmission of, and response to, each impulse of alternating current a short interval of time; so that, on calls completed through an intermediate toll ofiice, the a1ternat- 'ing current signal impulses transmitted through the intermediate office to a succeeding office prior to the opening of the talking connection at the intermediate office will be ineffective to cause a signal response at the succeedingoffice.

A feature of the invention is a signaling system in which signals consist of impulses of alternating current of each of two particular frequencies in succession, the signaling apparatus at the ends of a line over which the signalsare transmitted being arranged totransmit one or more disconnect signals on an outgoing call, to transmit a disconnect acknowledgment signal on, an incoming call responsive to the receiving of a disconnect signal and repeat the transmission of the disconnect acknowledgment signal as long as disconnect signals are being received, and to end the transmission of disconnect signals on an outgoing call when a disconnect acknowledgment signal is received. According to this feature the disconnect signal first transmitted from the calling end of the line consists of an impulse of cur-- rent of a first one of two particular frequencies followed "by an impulse of current of the second of these frequencies, theimpulse of the first frequency being long enough to enable response to the impulse of the second frequency and the impulse of the second frequency being long enough to effect the required release operations at the called end of the line; and each of the succeeding disconnect signals transmitted consists of impulses of current of saidfirst and second frequencies alternately for a plurality of cycles, the last impulse of the second frequency in each cycle being long enough to effect the required release operations at the called end of the line.

Another feature of the invention is an arrangement in a system ofthe above described type which insures release of the line over which signals are being transmitted, in case the line is seized simultaneously at both ends. f

A clear and complete understanding of the invention will be facilitated by considering a system embodyin the invention and its features, one such system being represented schematically in the drawings which forma part of this specification. The invention is not limited in its application to the particular system and circuit arrangement shown in the drawings, but is'applica'ble generally to any voice frequency signaling system. 7

Referring to the drawings:

Figs. 1, 2 and 3 represent a first toll oflice comprising an operators position OP and cord CDl, an outgoing trunk circuit OTI, a two-way trunk circuit TWTI, a toll selector TSI, an incoming trunk circuit ITI, a toll line circuit TLCi terminatin a toll line 'TLI, asignal transmitting circuit ST! and a signal receiving circuit SRI;

Figs. 4, 5 and 6 show a second toll office, similar to that of the first toll office; and

Fig. 7 shows the relative position in which each of Figs. 1 to 6 is to be placed to form an operative arrangement.

Reference may be had to the patent to King Miller No. 2,208,765, granted July 23, 1940, fora detailed description of the operators position and cord circuits provided in each of the two toll ofiices for interconnecting calling and called lines and trunks and controlling the completion of toll calls. The toll line circuits TLCI and TLCZ, outgoin trunk circuits OTl and GT2, incoming trunk circuits ITl and IT2, the two-way trunk circuits TWTl and 'I'WT2 and the toll route selector circuits TSI and TS2 are all similar to corresponding circuits described in detail in Patent No. 2,306,236Jgranted to J ."GfWalsh December 22, 1942. The toll line circuit TLCI comprises a repeating coil 200, a directionally selective coil 2l3l and a normally operated cut-off relay 202 connecting the repeating coil 20!] to the talking conductors of the outgoing trunk circuit OTI. The coil 2lll transmitssignaling current incoming over toll line TLI-to the signal receiver SRI but prevents the transmission of-anysubstantial portion of the signaling energy outgoing from transmitter STI to the signal receiver SRl. Further protection from interference due to si nal transmitted by signal transmitter STI may be secured by theinsertion of filters, tuned to suppress frequencies Fl and F2, between coil l and the input transformer 3M of signal receiver SRI. The normally operated cut-ofi relay 202 is released, to open the talking connection between the toll line TLI and the trunk circuits OTl and TWTl, whenever signals are being sent by transmitter STI or received by signal receiver SRI. The toll line circuit TLC2 is similar to the circuit TLCI, the reference characters of i like elements in these circuits having-the same tens and units digits. The operators positions are provided with dials for use in controlling the operation of incoming and auxiliarytoll route electors in other toll offices and for controlling the operation of selector and connector switches in local and community dial offices.

The si nal transmitter STI comprises relays 2l0, 220 and 230 connected in multiple to conductor I64 and controlled by relay I40 of outgoing trunk circuit OTI. Relay 2 l 0 is a polarized relay having a biasing winding. The operating winding of relay 2") is connected to conductor I64 througha delay network Nl; whereby-the operation and release of relay 2"] are each delayed for an interval of about .05 second. The network Nl is a multisection low pass filter so that the delay thus introduced is substantially constant for varying rates of dial impulse transmission; Relay 220 has two windings connected in parallel between conductor I54 and condenser 223, rectifiers 22l and 2-22 being connected in series with the windings. of relay so that one winding is operatively energized while condenser 223 is chargin responsive to operation of relay I40, so that the other winding is operatively energized while condenser 223 i discharging responsive to release ofrelay l40,-and so that both windings are energized in series to maintain the operation of relay 223 for an interval of about .1

second following the operative energization of either one of the windings. Relay :3!) is a slowto-release relay having a release time of about -.025 second. The signal transmitter STI further comprises relays 224 and 225 which are controlled by relay 220 and cooperate wtih relay 2IIJ to effect the transmission of seizure, selective, ringing, supervisory and disconnect signals. The transmitter STI further comprises relays 249, 24 l, 242, 244 and 245 for transmittin repeated disconnect signals and for transmitting disconnect acknowledgment signals. The signal transmitter STl further comprises a source 250 of alternating current of a first signaling frequency Fl and a source 260 of alternating current of a second signaling frequency F2 for use in sending signals jthe transmitter circuit STl, the reference char- 10.

acters of like elements in each circuit having the same tens and units digits. The two frequencies Fl and F2 used for transmitting signals over toll line TLI in one direction are different from the two frequencies F3 and F4 used for transmitting signals in the other direction. For instance the frequencies 600 cycles and 1500 cycles y be used to transmit signals in one direction and the frequencies 700 cycles and 1700 cycles may be used to transmit signals in the opposite direction.

The signal receiver SRI comprises an input transformer 3M, a volume limiting amplifier 304 and an output transformer 305 through which all current incoming over line TLI is transmitted. The receiver further comprises volume limiting detector 3|[l and polarized relays 3M and 3! responsive to signaling current of frequency F3, and a volume limiting detector 320 and polarized relay 324 responsive to signaling current of frequency F4. The condenser 3H and inductive coil 3l2 constitute a network which is tuned to have a high impedance at the signaling frequency F3; and the condenser 32l and inductive coil 322 constitute a network which is tuned to have a high impedance at the signaling frequency F4. Each of relays 3l4, 3l8 and 324 has a biasing winding and an operating winding, the left contact of each of these relays being normally closed as shown in the drawing due to the'normal energization of theoperating winding. The signal receiver SRI further comprises normally operated relays 333 and 340, slow-to-release relays 342, 350 and 355 and a polarized relay 360 for responding to seizure, selective, ringing and answering supervisory signals. Relay 330 is normally energized under control of each of relays 3M and 355; and relay 340 is normally energized delay network 362, similar to delay network Nl of signal transmitter 'STl, delays the response of relay 360 for .05 second. The left contact of relay 350 is normally closed as shown in the drawings, and this contact is held closed as long as the biasing winding is energized. This delay prevents false signal operations on tandem switched connection due to transmission of all of the first impulse and part of the second impulse of a signal before the cut-off relay releases to open the talking connection at the intermediate switching point. i The signal receiver SRl further comprises relays 310, 38l and 382 for response to disconnect and disconnect acknowledgment signals; and comprises relays 390 and 395. for control of thesignal transmitter STI and signal receiver SRI according to whether the call is an outgoingor an incoming call. Relay and 388, which network tunes relay 38l for re-- sponse to disconnect acknowledgment signals; The signal receiving circuit SR2 is similar to the receiving circuit SRI, the reference characters of like elements in these circuits have the same tens and units digits.

Further explanation of the invention will be facilitated {by describingin detail the circuit operations involved on calls over toll line TLI. Assume that a call incoming tothe operators position OP has been answeredwith answering plug API of cord CDi, that thecalling plug CPI of this cord has'been inserted in jack J I of outgoing trunk circuit OTI, and thatthe call is one which may be completed through intertoll trunk line TLI. Relays I04, H and I40 are operated, responsive to insertion of plug CPI in jack JI, in the manner described in the aforementioned patent to King-Miller. The operation oirel'ay H0 connects ground to conductor I68 to operate and lock relay I004 of two-way trunk circuit TWTI. The operation of relay I40 closes a circuit including conductor I64 for operating relays H0, 220 and 230 of the signal transmitter ST! and for operatively energizing the upper winding of relay 390 of signal receiver SRI. Relay 390 closes a locking circuit through its lower winding, which circuit is under the joint control of relays 310 and 382. It is assumed that relay 3 'I'0 is normal and relay 382 operated when relay I40 operates, this being the case when no incoming seizure signalhas been received at the time thatrelay 390 operates. The operation of relay 39il prepares thesignal transmitting circuit for therepeated transmission ofa disconnect signal when the connection is released; prepares the signal receiving circuit for response to supervisory and disconnect acknowledgment signals; and prevents the operation of relay 245, so that the signal transmitter isuna'ble to send a disconnect acknowledgment signal on anoutgoing call. If the oflice at the distant end of line ILI includes links arranged to send a stop dial signal whilepreparing to receive dial signals, conductor 39! is connected to ground and the operation of relay 390 connects this ground through contact 392, back contacts of relays 395 and 3I0,'to conductor I10,

to operate relay I asa stop dialxsignal. The operation of relay I05 opens the circuit through the lower, high resistance winding'pf relay I04 thereby to effect the extinguishing of a supervisory lamp at the operatorsposition to indicate that dialing should not be started. This local circuit operation of relay I05 in advance of the i receipt of a stop dial signal over line TLI insures the extinguishing of the operators supervisory lamp early enough to be effective as a stop dial signal. If the called office is of such a character that a stop dial signal is not required, conductor MI is not provided. The operation of relay 220 responsive to the connection of ground to conductor I64 causes the operation of relays 224 and 225 in succession and the release of cut-off relay 2020f the toll line circuit TLCI. Relay 202 is normally energized in a circuit including the inner upper back contact of relay 245, inner upper back contact of relay 240, conductor 226,, uppermost front contact of, normally operated relay 330, r inner upper front contact of normally operated relay 340, conductor 23!, and through back contacts of relays 225, 224 and 220 toground. The release of relay 202 disconnectsthe, talking conductors I6I and I62 of outgoing trunk circuit OTI from repeating coil 200 oftoll line TLI,

connects the upper rwinding ofl' coil 206 of signal transmitter STI to repeating coil 200,-andcon- 'nects ground to conductor N58 to hold relay i004 of two-wayrtrunkqcircuit 'I'W'II. The operation .of relayv 230 connectsi ground .to c0ndu0to1f232 and so also does the operation of relays-224 and 225 thereby to provide a locking ground connection for relay 245% and when relay '245 operates as hereinafterdescribed. l The operation of relay 224 connects ground to conductor 20'! thereby completinga circuit through the lower winding :of transformer 206, outer upper'back contacts of relays'245 and 240,-1eft contact of relay 2I0 and the source 250, therebyinitiatingthe transmission of signaling current of frequency FI through transformer 206; back contacts of relay 202, coil 200 and toll line TLI; About, but not 'less' than, .025 secondxafter the release of "relay 202 and operation oftrelay 224, relay 2 I0 operates. The operation of relay 2 l0" disconnects source Relays 2 I 0 and after its operation; causing the successiverelease of relays 224'and 225unles's the transmission of dial impulses is initiated before either or both -01 these relays hasnreleaised. *Therelease time of relay 224 is approximately .05 second and the release time of relay 225 is approximately .2- second. When relay 224 releases, it disconnects ground from conductor 20'! thereby ending the transmission of current offrequency F2 through transformer 206 and'coil 200. When relay224 trunk circuit OTI at position 01?; 7

Consider nowthe operationof the signal receiver SR2 responsive to theseizuresignal. The voltage generated across thesecondary windings of coil 400 due to current incoming over toll line TLI causes a current through the left windings of coil 40I, conductors 403 and 404, condensers 5M and the primary winding oftransformer I of signal receiver SRZF The secondary winding of transformer 50I is connected, through condenser 503 and resistor 502 in paralleLto the grid of amplifier tube 504. The anode-cathode circuit of tube 504 includes the primary winding of transformer 505; and this transformer hastwo secondary windings through which the. output of tube 504 is separately applied to the grids of detectontubes 5I0 and 520. The amplifier 504 and detectors 510 and 520 have 'volume limiting characteristics due tothe voltage drop introduced by the resistors 502, 506 and 508, respectively.

The volume limiting action of tubes 5l0 and 520 prevents false signal operations from voice or interference currents since the current through theoperatin g windings of relays 5I4and5l8 is of Signaling frequency FI; andsincethe current through the operatingwinding of relay 524.:is not quency Fl ends, relay l4 releases.

of transformer 50l is substantially all of signaling frequency F2. The volume limiting action of tube 504 prevents overloading of tubes 5|0 and -520 thereby helping to prevent false operations of relays 5l4, 5l8 and 524. The anode-cathode circuit of tube 5l0 includes a tuned network, consisting of condenser 5H and. inductance 5l2, and the operating windings of relays 5! and 5l8;

and the anode-cathode circuit of tube 520 includes a tuned network, consisting of condenser 52l and inductance 522, and the, operating winding of relay 524. Condensers 5I3', 5|! and'52 3 are connected in parallel with the operating windings of relays 5l4, 5l8 and 524 respectively to prevent these relays from tending to operate responsive to alternating current of the signaling frequencies and to provide a low impedance path for other frequencies.

When a seizure signal is received by signal receiver SR2, the impulse of current of frequency Fl effects the operation of relay 514. Although the lower, operating winding of relay 5I8 is in series with the lower, operating winding of relay 5l4, relay 518 does not respond due to the high normal bias of relay 5l8. The operation of relay 5l4 closes a circuit from ground through the left contact of relay 5| 4, conductor 5; and outer upper back contact of relay 542 for operating relay 550; When the impulse of current of fre- The release of relay 5| 4 opens the operating circuit of relay 550; but being slow to release, relay 55B remains operated for about .0l5 second; and, during this time, the impulse of current of frequency F2,

which follows the impulse of frequencyFi effects the operation of relay 524. The operation of relay 524 before relay 550 releases, closes a looking circuit for relay 550 which circuit is traced from battery through the winding of relay 550, resistor 55!, lower front contact of relay 550, conductor 52,6 and the left contact of relay 524 toground. With both of relays 524 and 550 operated, the normally operated relay 540 releases. The release of relay 540 causes the release of the normally operated relay 402 of line circuit TLC2 and causes the operation of relay 542. The release of relay 402 disconnects coil 400 from talking conductors 66! and 662 and connects the upper winding of transformers 405 to coil 400. Relay 542 locks through its inner lower front contact under control 'of relay 510, closes a circuit for energizing relay 555, and connects ground to conductor 610 to effect the operation of relay 103 of two-way trunk circuit TWT2. The circuit for operating relay 555 includes the inner upper front contact of relay 542 and the left contact of relay 524; and relay 524 may or may not remain operated long enough to effect the operation of relay 555. The circuit for operating relay 103 is traced from battery through its winding, back contacts of relays l0! and I04, conductor 669, back con-tact of relay N0 of outgoin trunk circuitOTZ, conductor 610, back contact 51'! of relay 510, front contact 543 of relay 542, and the normally closed right contact of relay 560 to ground.

of current of frequency F2 ends, relay 524 rereduced sufiiciently to effect the operation of this relay unless the current in the primary winding leases. The release of relay 524 causes the reoperation of relay 540, the release of relay 550 and the release of relay 555, if operated; but relay 542 remains operated and the connection of ground to conductor 610 is maintained. The .reoperation of relay 540 causes the reoperationof relay 402, whereby coil 400 is again connected to talking conductors GM and 662. i

It is to be noted that the operation of relay 542 responsive to the seizure signal opens the operating circuit of relay 550: thereby disabling the operation of the signal receiver responsive to any signal or interference current beginning with frequency Fl, thereby reducing the possibility of false operations of signal receiver SR2 after response to a seizure signal. The aforementioned operation of relay 542 also prevents the operation of relay 590 when battery is connected to conductor 664 to effect the transmission of a supervisory signal. Relay 542 is sufliciently slow in releasing so that it will remain operated during the receipt of dial impulse signals or a rering signal.

The aforementioned release of relay 402 effects the connection of ground to conductors G68 and so also does the operation of relay 103; whereby the toll line TLI is marked busy at the'multiple jacks at which trunk circuit 0T2 terminates and at the terminal banks of toll selectors such as TS2 having access to the toll line TLI,- as set forth in detail :in the aforementioned patent to J. G. Walsh. The aforementioned operation of relay 103 closes a circuit for operating relay 105 of two-way trunk circuit TWT2 and the line relay 751 of selector TS2. This circuit is traced from battery through the windin of relay 151, lowermost back contact of relay 160, resistor 153,

thence in simplex through resistors NH and 162, back contacts of relay 160, front contacts of relay 103, conductors l0] and 102, back contacts of relay 630 of outgoing trunk circuit OTI, both windings of retard coil 660, back contacts of relay 620 and resistors SH and 622, thence through conductor 661, uppermost front contact of relay 103 of two-way trunk circuit TWT2, winding of relay 105, back contact of relay 104, conductor 669, back contact of relay H0 and through conductor 610, back contact of relay 510, front contact of relay 542 to ground at the right contact of relay 560 as described for operating relay I03. Theoperation of relay 105 closes a circuit for operating the slow-to-release relay l0! and closes a holding circuit through resistor 106 for bolding relay I03 operated after relay 10! operates. Relaylfll also connects ground to resistor 106 to hold relay 103 operated, opens the operating circuit of relay I03, and connects ground to conductor 153' to hold the switch TSZv after it has selected a trunk as hereinafter described. The operation of relay l5! prepares the selector TS2 for-selective response to the first train of dial impulses incoming to signal receiving circuit SR2 in the manner described in the aforementioned patent to J. G. Walsh.

If it is necessary to transmit a stop dial signal'to the calling operator, as would be the case where a finder switch operates to extend a calling trunk line to an incoming selector, the stop dial signal is transmitted in the manner hereinafter described for the transmission of an ofi hook signal. And when dialing may be started, a start dial signal is transmitted in similar manner to the transmission-of an "on hook signal as hereinafter described.

mission of current of frequency F2.

When the operation at position OP dials the first digit of the code which identifies the called toll or local ofiice, relay I40 is alternately released and reoperated as many times as there are units in the digit dialed under the control of the impulse contact of the dial at position P. Each release and reoperation of relay I40 effects a corresponding release and reoperation of relay 2m, the response of relay 2! being delayed in each case for about .050 second by the network NI Relay 22B is energized by the discharge of condenser 223 when relay I40 releases'in response to the first impulse of the digit; and sincerelay I40 reoperates after each release during dialing before relay 220 has released, relay 22G remains operated during response of relay M9 to the impulses of each digit and releases about .1 second after relay I40 reoperates at the end of the last impulse of each digit dialed. Thus relay 220 operatively energizes. relay 224 at the beginning of each train of dial impulses; and relays 224 and 225 are operated and remain operated during the transmission of each train of dial impulses. The

operation of relay 225 causes the release of relay 252. Relay 230 releases each time relay I40 releases responsive to dial impulses. Since the release time of relay 235 is not less than .625 second, relay 230 will not release before relay 224 operates responsiv to the first dial impulse of a digit, thereby pr ve tin the. l e er tio of relay310 of signal receiver SR! in case relay 225 is operated at the time relay 233. releases respon sive to the firstimpulse of a series as is the case when the first digit is dialed as soon as the trunl; OTl is seized. Since relay 2) is operatedat the time the first digit is dialed, the operation of relay 2-24 or, if relay 225 has not released prior to the dialing of the first digit, the release of relay 2311 connects ground to conductor to initiate the transmission of signaling current of frequency F2 over toll line TLI. When relay 210 releases about .050 second after relay I40 releases responsive to the first dial impulse and not less than .025 second after the connection of ground to conductor 201', the source 250 is disconnected from, and the source 250 is connected to, transformer 205 thereby ending the impulse of cur-- rent of frequency F2 and initiating the transmission of current of frequency Fl overtoll line TLI. When relay E reoperates. at theend of the first impuls of a digit, source25fl is disconnected from, and source 260 is connected to, transformer 205 thereby ending the transmission of current of frequency FI and initiating trans- The release and reoperation of relay 2E0 responsive to each succeeding impulse of the train effects theztransmission of an impulse of current of frequency :Fl followed by current of frequency F2. When relay 2) remains operated after receiving the last dial impulse of a digit, relays 220, 224 and, 225 release in succession, the release of relay 224 being effective to end th transmission of current of frquency F2 in the manner above described for terminating the current of this frequency at the end of the seizure signal. Inlike manner,::the dialing of each succeeding digit ofsthe toll route code, and also the dialing of eachdigi-t of a called subscriber number, by the operator at position OP causes the transmission of a train of-dial impulse signals by the signal transmitter "S'Il over toll line TLI'. At the end fthe diali-ngfof each digit, the conditions existing in signal trans; mitter ST! are the same as they'werefollowing transmission of the seizure signal;

v iveresponsivetcthe'reductionlof cur; rentin i lower, w nd ng he d i animp s' of, curren; of afrequency l l is received .oven toll. line be forejrelay 555relea'ses. When theim pulse of; currentof frequency F2 ends, relay 524: releases; but, being slowlto release, relay 55,5.re'5

mains operated Ior abouthql5fsecond after relay fltre ea es The im ulse-1 O urrent or fre-i asar which qllews th mr l ir unw F2 hem -relay r555 r l ases s i h 'r tier of be lie -re ays 5 .4. a d 518! The oper'ation d relay- 5. l 4 beiorerelay 555 releases, closes-a holdng; circu t ry l r 5 5;; i ir it is ced from battery through the winding oI-relay 5.55, resistor 556, lowerfront contact ofr elay 555, conductor;5l6 and the leftcontact of relay 5M. to ground. With relays 5M and 555 both operated, the normally operated relay 530 releases; and-it cannot-reoperate until relay 555 releases. 'The release-of relay 530 connects conductor 525 to ground at a back contact of relay 590 to prevent the release of relay 540; and causes the release of relay 452, whereby coil 400 is disconnected from talking conductors 6 5! and 552. With relay 530 released and relay 555 operated, the upper winding of relay 550 is deenergized, thereby rendering relay" 560 operatively responsive to operations of relay 518. With relay 5l8 normal, the lower winding of relay 560 is energized in one direction in a circuit traced from battery through resistor 564, delay network 562 and lower winding of relay 550 in series, conductor 565, lowermost back contact of relay 510, right contactof relay 51 8, and inner lower back contact of relay 510 to ground. With relay 5 I 8 operated, the lower winds ing of relay 560 is energized in the other direction in a circuit traced from battery through resistor 563, delay network 562 and lower-winding of relay min. se esconductor 565, ba k c nta t 4 of elay 9, le t contact of relay ,to ound at the innerlower back contact of relay 510. c The response ofrelay 560 toeach operation and each release of relay-518 is delayed for about .05 sec- 0nd by the delay network 562. Thus the operation of relay 5l8, responsive to the impulse of cure rent of frequency Fl representing the first dial impulse or a digit, causes the operation of relay 560 and the release of relay 518, at the endof this impulse, causes the release of relay 560., The

impule of current of irequency FZ representing the closing of the dialingcircuit at theend ofthe first impulse, causes the reoperation of relay 524. Each succeeding impulse of current of frequency Fl, representing a dial impulse of the first digit dialed, effects the operationof relays 514 and 5 l8; and each succeeding impulse of current of frequency F2 representing the closing of the dialing circuit at the end of a dial impulse efiects the operation of relay524. Each operation of relay 5 holds relay 555 operated, each operation and release of relay 518 causes the-operation and release .of relay560, and each operation of relay 5,24 hold relay 555 operated. Thus relay 555 is operwind n 1 e a t ender th ated .due" to the operation of relay- 524,"atthe beginning of each train of dial impulse signals, andis' held operated during the response of relay560 .toatrain of dial impulse signals; and whilerelay 555 is operated; relay 530 cannot-reoperate; The release ,of relay 524 at the end; of

the impulse of current of frequency F2 which is received for about .1 second at the end of a train of dial impulse signals, causes the release of relay 555 followed by the reoperation of'relay 530.

.Each operation and release of relay 560 responsive to the firsttrain of dial impulse signals trans rnitted over line TLI disconnects ground from and reconnectsground to conductor 610, thereby causing the release and reoperation of relay T05 of two-way trunk circuit TW'I2 and release and reoperation f: relay I51 of selector TS2 responsive to each dialimpulse signal of the train. -Being slow to release, relay I01- of two-way trunk circuit TWTZremains operated duringthe transmission of dial impulses over conductor 510.- The selector PS2 is thereby selectively operated under the control of 'relayfI5'Ito select a level corresponding to the firstdigit of'thetoll route code and to select an idle trunk connected to this level; relay I60 being'thereupon-operated to extend the connection through brushes 1! I, 712, H3- and 114 to the selected trunk, all in the 'manner described in detail in the aforementioned patent to J. G. Walsh j -Thetrunkselected by selector TS2 responsive to the first digit dialed by the operator at OP may extend to an auxiliary route selector similar to selector TSZ, to another toll line, to atrunk to a local omce, or to an incoming trunk circuit IT2,at atoll operator's position. Ifthe operator at position OP dials additional digits, the signal transmitter STI transmits a train of dial impulse signals corresponding to 'each digit dialed and the signal receiver SR2 repeats these dial impulses over conductor 6'I0'to effectthe selective operation of additional selectors'through which the connection is extended. When dialing is'completed and the position dial key-(not shown) is restored to normal, relay I30 is operated toconnect the talking conductors of cord CD'I, through front contacts of this relay and the talking conductors II andIBZ-Qftrunkcircuit 'OTIto the repeating coil200as described in the aforementioned King-Millerpatent.

. 'Assuming that the 'call is routed to a tolloperatorthrough an incoming trunk circuit 1T2 connected to terminals in the bank of selector TSZ, the operation of relay 1600f selector-T82 closes a circuit through brushes HI and I12 for operating relay 805. The operationof relay--805=lights the lamp L8 associated with jack J8. When'an operator answers the call by inserting the plug of a cord in jack J8, relay 834-is-operated as described in detail in the aforementioned patent to J. G.-Walsh. The operation of relay 834 connects ground through conductor 804, brush II4 and-a front contact ofrelay 160, to'operate relay III of two-way trunk circuit T'WTZ. The operation of relay III disconnects ground from and connects signaling battery to conductor 554- to effect the transmissionof an answering supervisory or off hook signal by transmitter S'IZ over line TLI to the calling toll oflice in similar manner to thatiin which the .seizure signal was transmitted by'signal transmitter STI responsive to'the connection of' signal batteryto conductor I64-as hereinbefore described. The foffhook signalthus transmitted consists of an impulse of current of frequency" F3 followed by an impulse relay 390 of signal receiver SRI. The signal receiver SRI responds to this signal in similar manner to that in which SR2 responds to a seizure signal as hereinbefore described; except that, since relay I10 has been operated in outgoing trunk circuit OTI, the connection of ground to conductor I70 efiects the operation of relay I05 of outgoing trunk circuit OT I instead of operating relay I003 of two-Way trunk circuit 'I'WTI. The operation of relay I05 disconnects ground from the lower Winding of relay I04, whereby the supervisory lamp CS is extinguished. In case dialing has not been completed, when relay I05 operates, the sleeve conductor of plug CPI is connected to relay 22 instead of to lamp CS; and in this case the marginal relay 22 releases to extinguish the dial pilot lamp DP as a stop dialing signal.

If the answering operator disconnects the answering plug from jack J8, relay 834 releases, causing the release of relay 1! I of two-way trunk circuit TWTZ. The release of relay III reconnects ground to conductor BS4, whereby the signal transmitter SP2 is operated to transmit an on hook signal. Whenbattery is disconnected from, and ground is connected to, conductor 604 by the'release of relay III, relay 420 is operated for about .1 second and relays M0 and 430 are deenergi'zed, the release of relay 430 occurring at the end of an interval of about .025 second and the release of relay 4) occurring at the end of an interval of about .050 second. The operation ofrelay 420 causes the release of relay 402 and the successive operation of relays 424 and 425. The operation of relay 424 closes a circuit from ground, through the lower winding of transformer 406, back contacts of relays 445 and 440 and left contact of relay 4I0 to source 450 of currentof frequency F4; and with relay 402 re- Since relay 590is not operated, the release of re-,

lay 424 does not cause the operation of relay 510 of signal receiver SR2. When relay 425 releases, ground is disconnected from the lower winding of transformer 406 thereby ending the impulse of current of frequency F3, the duration of this impulse being about .3 second. When relay 425 releases, it again closes the normally closed circuit for operating relay 402.

I When a start dial or on hook signal is received by signal receiver SRI, relay 324 operates responsive to the current of frequency F4. The operation of relay 324 closes a circuit from ground through its right contact, conductor 326, and'inner upper front contact of relay 342 (this relay having been operated and locked responsive to the off hook signal) for operating relay 355. Relay 355 connects ground to resistor 3I9 to reduce the bias of relay 3I8 and opens the operating circuit of relay 330. When the impulse of current of frequency F4 ends, relay 324 releases.

Relay 355 is slow to release and the impulse of current of frequency F3, which follows the current of frequency F4 before relay 355 has time to release, causes the operation of both of relays 3I4 and 3I8. When relay 3I4 operates, it holds relay 355 and causes the release of relay 330, thereby rendering relay 360 responsive to the energization of its lower winding under the control of relay 3I8; The operation ofrelay 3I8 causes the operation of relay 350, the network 352 being effective to delay the response of relay 360 for about .05 second. When relay 360 010- erates, it disconnects ground from conductor I10, to cause the release of relay I05; and connects ground through its right contact and a front contact'of relay 340 to resistor 34I, thereby short-circuiting the winding of relay 342; The release of relay I05 connects ground to the low resistance Winding of relay I04, causing the cord supervisory lamp OS to light. Since the impulse of current of frequency F3 continues for .3- second, relay342 releases; thereby preventing the reconnection of ground to conductor I when relay 360 releases "at the end of the impulse. The release of relay 330 closes a circuit for operating relay 395 and opens the operating circuit of relay 390. Relay 395 looks through its lower winding under control of relays 310 and 382. If the signal is an on book signal with conductor 39I omitted or is an on hook signal following'an ofi' hook signal, ground is disconnected from conductor I10 as soon as relay 360 operates; but in the case of a start dial signal, it is to be noted that ground is not disconnected from conductor I10 until relay 395 operates to end the local stop dial signal condition, established responsive to seizure of trunk circuit OTI.

If the calling operator'operates the ringing key (not shown) in the cord CDI to recall the answering operator, relay I40 is released and reoperated to effect the transmission of a. rering signal. The signal transmitter STI and signal receiver SR2 are operated in the same manner as if the digit I had been dialed by the operator, the connection of ground to conductor 610 being thereby momentarily opened to give the answering operator a recall signal in the manner described in the aforementioned J. G. Walsh patent.

Disconnect When the calling operator removes the plug CPI from jack JI, relays I40, I30, H0 and I04 of outgoing trunk circuit OTI release, as described in the aforementioned patent to J. G. Walsh. At that time, relay 342 may or may not be operated, depending upon whether an on hook or an off hook signal condition exists in the signal receiver SRI. The release of relay I40 disconnects signaling battery from and connects ground to conductor I64, causing the release of relay 2I0 at the end of an interval of .050 second, causing the operation of relay 220 for .1 second and causing the release of relay 230 at. the end of an interval of .025 second. Theoperation of relay 220 causes the release of relay 402 and the successive operation of relays 224 and 225. The operation of relay 224 closes a circuit through the lower winding of transforme 206 and the source 260 of current of frequency F2; and with relay 202 released, current of frequency F2 is transmitted over toll line TLI. When relay 2I0- releases, the lower winding of transformer 206 is transferred'from source when the seizure signal was sent out, the release of relay 224 closes'a circuit for operating relay 310 of signal receiver SRI. This circuit is traced from ground at the outer upper back contact of relay 230, through the lower back contact of relay 224, inner lower front contact of relay 225,

conductor 221, inner upper front contact of rea lay 390, and winding of relay 310 to battery. Relay 310 locks under the control of relay 390; opens the locking circuit of relay 342, if this relay is operated; transfers the contacts ofrelay 3 I8 from relay 360 to relay 38I; connects ground to conductor 325 to hold relay 340 operated, in-

dependent of relay 324; disconnects ground from conductor I10, if connected thereto; and prevents the operation of relay I003 of two-way trunk circuit TWTI in 'case the off hook signal condition exists in signal receiver -SR.I at the time relay IIO of outgoing trunk circuit OTI releases. When relay 225 releases, it ends the transmission of current of frequency Fl; Thus, a disconnect signal is transmitted consisting of an impulse of current of frequency F2 for about .025 second followed by an impulse of current of frequency FI for about .3 second. If a disconnect acknowledgement-signal is not received over toll line TLI from the called office before relay225 releases, relay 240 is operated "by the closure of a circuit which includes the lower back contact of relay 225, conductor 228, a front contact of relay 390, a front contact of relay 310 40 and a front contact of the normally operated relay 382. The operation ofrelay 240 maintains the deenergization of relay 202, connects ground to the lower windingof transformer 206, and transfers the control of the energization of this winding from relay 2I0 to relays 2H and 244. The operation of relay 240 opens the normally closed circuit for" operating relay HI and connects the 20-cycle source 243 to the winding of relay 244, whereby relay 244 is operated and released twenty times per second to effect the alternate transmission of current of frequencies F2 and FI through coils 206 and 200 over toll line TLI until relay 2 releases. When relay 24I releases, .25 second after the operationfof relay 240, it connects source 250 to the lower winding oftransformer 206 to effect the transmission of current of frequency FI through coils 206 and 200 and also opens the normally closed operating circuit of relay 242. The release of relay 242 about.25 second later' closes a circuit for reoperating relay 2. The operation of relay 24I ends the transmission of current of frequency FI and again initiates the alternate transmission of current of frequencies F2 and FI under the control of relay 244 and 20-cycle.

source 243. The operation of relay 24I also closes the circuit for operating relay 242. Thus, relays 2, 242, 244 and the source 243 effect the cyclic transmission of current ofnfrequencies FI and F2 alternately at a 20-cycle rate for 25second followed by current of frequency FI for .25 second, the cycle being repeated until relay 240 releases due to the receipt of a disconnect acknowledgement signal as hereinafter described.

When the disconnect signal is received by signal 260 to source 250, therebyending the transmis-; sion of current of frequency F2 and initiating the receiver SR2,:relay 524-, operates and releases re'-'. sponsive to each impulse ofcurrent of frequency F2 and relays 5,1l4-and15l8 operate and release-re sponsive to each'impulse of current-of frequency;

disconnects ground. fronrconductor 610 tolcause the release of relay 155 of. two-way trunk circuit TWT2 and relay- 805' of trunk circuit 1T2; and since the impulseof current of frequency FI con-' tinues for about .3'second. relay 560 remains operated for along enough interval to cause the release of relay 542. .The disconnection of ground from conductor 61!] due to the operation ofrelay 560 and release of relay 542 causes the successive release of, relays-ml and 10301 two-way trunk.

circuit TWT2. -If and when relay llllreleases; it disconnects ground. from-;conductor:-153 thereby causing the-return of selector T82 and trunk circuit 1T2 to normal as described in'the aforementioned patent to J. G. Walsh. If'there isinterference, for instance due tothe alternate transmission of off hook and on hook signals or to the transmission of a busy tone, when an alltrunks-busy or called-line-busy condition is en:

countered 'in extending a connection, relay 560 .01?

signal receiver SR2 may not be operated fora long interval in response to the first long impulse of current of frequency Fl to effect the release of relay 542 and relays I01 and-103 of two-way trunk circuit TWT2. In this event, the operation of relay 560 responsive to one of the long impulses of current of. frequency Fl of a succeeding disconnect signal will be effectivezto cause the release of these relays. In any event, relay 555' is held operated and relay 530 remains released as' long as relays 5l4, 5l8 and 524-continue to operate and release'responsive to the disconnect signal transmitted over line TLl. Relays .5 l4 and *524 may-not continue to operate due to interference; but, when they do operateresponsiveto'the cycle alternations of the disconnect signal during a short 01f period of the interference; relay555' operates, relay 530releases-and' relay 462 releases: This opens the talking circuit to end the transmission of a busy tone and; allows the next long pulse of. current of frequency Fl to release relay 542.

If and when relay 560 is operated fora longenough interval torcause the release of relay 542; relay 445 of signaltransmitter ST2:-is-operated by the current in a circuit which-traced from the winding of relay 445 through-conductor 446, lowermost back contact of relay 590, a front con tact of the normally operated relay 582, lower back contact of relay, 542, to ground at-a back contact of relay 535. Relay 445 connects groundto one end of the lowerwinding of transformer 40B, connects the vother .end of this winding through contacts of relay 444 to source 460- of current of frequency F4, and connects the-20 cycle source 443 to the winding of'relay 444. The alternate operation and .release -of relay 444 twenty times per second effects the U alternate transmission of impulses of current of frequencies F4 and F3 through coils 406 and 400 over toll line 'I'Ll'as longas relay 445 remains operated. This constitutes a disconnect acknowledgment signal;

The disconnect"acknowledgment signal thus transmitted over line-TL! causes the operation of relay 324responsive to each impulse of current of frequency F4 and the operation of relay 314 and 3l8 responsive -toeach impulse of current of frequency F3. The operation-0f relay 324 causes the operation of relay 355 and the operation of relay 355 renders relay 3 l 8 operative. The operation of relay 3l4 closes a circuitthrough resistor 355 for holding relay355 operated. With both of relays 354 and 355 operated, relay 330 releases. The release of relay ,330 causes the release of relay 202. With rela 3l0 operated as described previously when the disconnect signal was sent, the operation and releaseof relay 3!?! twenty times per second causes the operation of relay 38l relay 38! being timed.as,jhereinbefore described. The operationof relay 38I short-circuits the winding of relay 382; and relay 332 releases about .1 second after relay 38! operates. 1 The release of rlay 382 causes therelease *of relay 240 thereby dis-- connecting ground-from transformer 256- to end thetransmission of the disconnect signal; Therelease of relay 240 also causes the rec peration of relay 24! if not then operated; and disconnects source 243 from the winding of reIay244, thereby ending the transmission of the disconnect signal over line TM. The delay inthe release of relay 382 reduces the possibilityof false release responsive to voice or other interferencecurrents and thus prevents the stoppingthe transmission ofthe disconnect signal before it has become efiective. The aforementioned release of-relay'382 alsocauses the release of relays 390 and 395. Relay 355 remainsoperated, relay 330 released and relay 315 operated,- as long as relays 3l4 and 324 continue to be alternately operated by the disconnect acknowledgment signal; and relay 31 remains operated and relay 382 released long as relay' 3 I B continues to be operated and 'released responsive to the disconnect acknowledgment signal. Relay 202 cannot reoperate while relay 330 remains released. Y l 7 When the transmission of the disconnect signal over line TLI ends, the contacts of-relays 5| 4, 5; and 524 return to, orremainin normal position, thereby causing the release of relay 555 and reope'ration of relay 530. The'reoperation of relay 530 opens the operating circuit of relay 445; and, if relays 425, 424-and 430 are normal as is the case if selector TSZ has restored to normal or the answering operator has disconnected from jack J8, relay 445 releases thereby :terminating the transmission of the disconnect acknowledgment signal over line TLI to the calling ofilce. The release of relay 445 also closes the circuit for operating relay 4fl2y and the operation of relay 452 disconnects ground from conductor 5613 thereby marking the toll line "ILl' as idle in the banks of toll selectors and at the operators outgoing multiple jacks.

When the transmission of the-disconnect acknowledgment signal ends; the contacts of relays 314, 3l8 and'324 return to or remain normal, thereby causing the release of relay 355and reoperation of relay 330 and causin the release of relay 3M and reoperation of relay 382. The reoperation of relay 335 causes the release of relay 310 and closes the circuit for operating relay 202. The operation of relay 262 disconnects ground from conductor I68 to mark-the toll line TLI as idle in the banks oftoll selectors and at the operators outgoing multiple jacks.

It is to be noted that relays202 and 402 are aesasrr each released as long as any signal is being transmitted over line TLI, thereby maintaining the connection of ground potential to conductors I68 and 863 to prevent seizure of line TLI at either end. In case the line TLl i seized at both ends simultaneously, the seizure signal transmitted from each end is received at the other end as a stop dial signal. If after a time a first one of the originating operators disconnects, a disconnect signal will be transmitted over line TLI, which signal will act as a start dial si nal at the other end. If in response to the start dial signal, the second of the two operators dials, the dial impulse signals transmitted over line TLl will not effect any selective operations at the end which sent out the disconnect signal because the signal circuits at that end are awaiting a disconnect acknowledgment signal. After a time the second of the calling operators will disconnect, whereupon a disconnect signal is transmitted over line TLI toward the end from which the transmission of a disconnect signal was first initiated. Thus, disconnect signals are being transmitted in both directions and the line IL! is marked busy at both ends.

At one end, relay 390 is operated, relay 292 is released, and relay 2 5i; is operated to eiiect the transmission of the disconnect signal consisting of impulses of current of frequencies Fl and F2; and in response to the incoming disconnect si nay, relay 355 is operated, relay 330 is released, and relay 310 is operated. At the other end, relay 590' is operated, relay 402 is released and relay 440 is operated to effect the transmission of V the disconnect signal consisting of impulses of current of frequencies F3 and F4; and in response to the incoming disconnect signal, relay 555 is operated, relay 535 is released and relay 510 is operated. Although neither end is trans mitting a disconnect acknowledgment signal, the disconnect signal from one end will be started before it is started from the other end; and, when the first long impulse of this signal ends, the impulses of the two frequencies then transmitted alternately twenty times per secondappears to be a disconnect acknowledgment signal at the other end. Assuming this otherend to be the end with which signal receiver SRI is associated, relay 38l is operated and relay 382released at the end of .1 second as hereinbefore described. The release of relay 382 causes the,

release of relays 390, 395 and 340 thus ending the transmission of the disconnect signal consisting of frequencies Fl and F2. Thetransmis-. sion of the disconnect signal consisting of frequencies F3 and F4 in the opposite direction over line TLI continues and during the receipt of a long impulse of current of frequency F3, relay 38l releases and relay 382 reoperates; whereupon relay 245 is. operated, to initiate the transmission of a disconnect acknowledgment signal consisting of frequencies FI and F2. When this disconnect acknowledgment signal is received by signal receiver SR2, relay 58l operates and relay 582 releases to end the transmission of the disconnect signal consisting of frequencies F3 and F4. When the disconnect signal is no longer received by signal receiver SRI, relay 355 releases, relay330 reoperates, relay 310 releases and relay 245 releases thereby ending the transmission of the disconnect acknowledgment signal consisting of frequencies Fl and F2. When the disconnect acknowledgment signal is no longer receivedby signal receiver SR2, relay 555 releases, relay 530 reoperates, and relay 510 releases. Re-

lays 202; and 402 are reoperated and the line TLI is again marked idle at bothends. In case the first 20-cycle alternation of frequencies. Fl and F2 should occur at thesame time that the first 20 cycle alternation of frequenciesF3 and F4 occurs, the transmission of the 'disconnectsignal will be ended at both'ends andno disconnect acknowledgment signal will be transmitted from eitherendif In the. case of a double seizure wherein only one of, relays 39 01and590 is operated, oneend is the." outward signalingcondition and the other in thein'ward' signaling condition. If disconnection occursfirst atthe end"conditioned as the outward end, a. disconnect signal willbe trans-V, mitted therefromtj At the endconditioned as the inward end, receipt of this ,disconnect signal causes the transmission of a disconnect acknowle edgment signal. Receipt of the disconnect acknowledgment signal at, the end conditioned as the outward end, stops theftransmissionof the at the end conditioned as the inward end,an

on hookfsignal transmitted therefrom, which signal acts as a start dial. signal at the end conditioned as the outward end. The operator at the'outward end may then dial without any selective operation being effected and when thisop erator disconnects the disconnect and disconnect acknowledgment signals will be transmitted as a regularly completed call as, hereinbefore described. I f What is claimed is: j 1. In a" signaling system, a trunk line, a trunk circuit connected to one end of said line, a link connected to said trunk circuit, means responsive to disconnection of said link from said trunk circuit for transmitting over said trunk line a disconnect signal consisting of current of one of two frequencies followed by current of the other frequency for a predetermined "interval of time and then in recurring" cycles current impulsesof said two frequencies altematelyfor a predetermined interval of time followed by current of said other frequency for a predetermined interval oftime, release control meansat'the other end of said trunk line responsive to a disconnect 'signalconsisting of current of 'saidone frequency followed by currentof said other frequency for a predetermined minimum interval of. time, signal transmitting meansat said other end of said trunk line rendered effective by said releasecontrol means responsive to a' disconnect signal to transmit over said trunk line. a disconnect acknowledgment signal consisting of current impulses of two particular frequencie alternately, control means at said one end of said trunk line for terminating the transmission of said disconnect signal, and means operated by disconnection of said link from said trunk circuit for rendering said control means operatively responsive to said disconnect acknowledgment signal.

2. In a signaling system, a trunk line, a trunk circuit connected to one end of said line, a link connected to said trunk circuit, means responsive to disconnection of said link from said trunk circuit for transmitting over said trunk line a disconnect signal consisting ofcurrent of one of two frequencies followed by current of the other frequency for a predetermined interval of time and then in recurring cycles current impulses of said two frequencies alternately for a predeterminedintervalfof time followed by current of said other frequency for a predetermined "interval of time, release controlmeans at the other end of said trunk line responsive to a disconnect signal consisting of current of said one frequency followed by current of said other frequency for a predetermined minimum interval of time, signal transmitting means at said other 'end' of said trunk line rendered effective by said release-control means responsive to a disconnect signal to transmit over said trunk Cline a disconnect acknowledgment signal consisting of current 'impulses of two particular frequencies alternately, control means at said one end of said trunk line for terminating the transmission of said disconnect signal means operated by disconnection of said flink 'from'said trunk circuit for rendering said control means op'eratively responsive to said disconnect acknowledgment signal, "and control means at said other end of -said trunk line for terminating the transmission of a disconnect acknowledgment signal responsive to termination of a disconnect signal incoming over said trunk line.

'3; ma signaling system, a trunk line, a -trunk circuit connected to one end'of said line, a link connected to said "trunk circuit, means responsive'to disconnection of said, link from said trunk circuit for transmitting over said trunk line a disconnect signal consisting or current or one of two frequencies followed by current of the other frequency for a predetermined interval of time and then in recurring cycles current'impulses of said two frequencies alternately for a predeter mined interval of time followed by currentof said other frequency for a predetermined interval of time," release control means atth'e other end of said trunk line responsive to a disconnect signal Consisting of current of said one frequencyfollowed by current of said other frequency for 'a predetermined minimum'interval of time, signal transmitting means at said other end of said trunk line renderede'fiective'by said release control means'responsive to a disconnect signal 'to transmit over said trunk line a disconnect 3- acknowledgment signal consisting bf current imteam? followed by current of said other frequency for a predetermined minimum interval of time, signal transmitting means at said other end of said trunk line rendered effective by said release control means responsive to a disconnect signal to transmit over-saidtrunk line a disconnect acknowledgment signal consisting of current impulses of two particular frequencies alternately, control means at said one end of said trunk line for terminating the transmission of said disconnect signal, means operated by disconnection of said link from said trunk circuit for rendering said control means operatively responsive to said disconnect acknowledgment signal, control means at said other end of said trunk line for terminating the transmission of a disconnect acknowledgment signal responsive to termination of a disconnect signal incoming over said trunk line, and means for marking saidtrun'k line busy at said other end as long as said disconnect acknowledgment signal is being transmitted therefrom.

5. In a signaling system, a trunk line, a trunk circuit connected to one end of said line, a link connected to said trunk circuit, means responsive to disconnection of said link from said trunk circuit for transmitting over said trunk line a disconnect signal consisting of current of one of two frequencies followed by current of the other frequency for a predetermined interval of time and then in recurring cycles current impulses of said two frequencies alternately for a predetermined interval of time followed by current of said other frequency for a predetermined interval of time,

- release control means 'at the other end of said pulses of two particular frequencies alternately;

control means at said one end of said Ytrunkline for "terminating the'transmiss'ion I of said disconnect signal, means op'erated by disconnection of said link Tfrom said trunk circuit for rendering said control means -opert'itively responsive to said disconnectacknowled'gment signal, controlmeans at said other end of said trunk'line'for terminat ingthetransmission ofa disconnect acknowledgment signal responsive to termination of a disconnect signal incoming over said trunk line, and

means for markingsaid trunk line busy at said 7 one end as long as said disconnect acknowledgment signal is 'be'ingreceiived.

4. In a signalingsystem, a'trunk line, .a trunk circuit connectedto one end of sai'd'line, a link connected'to said trunk circuit, means responsive to disconnection of said linklfro'm said trunk cir cuit for transmitting over said trunk line a disconnect signal consisting of current of one of two frequencies followed by current of the other frequency for a predetermined interval of time and then in recurring cycles current impulses of said two frequencies alternately for a predetermined interval of time followedfby 'current'of said other frequency for iafp'redetermine'd interval of time, releasecontrol means-at the other :end of said'trunk line'lr'esponsive to a disconnect signal "consisting ofcurrent of said one frequency trunk line responsive to a disconnect signal consisting of current of said one frequency followed by current of said other frequency for a predetermined minimum interval of time, signal transmitting means at said other end of said trunk line rendered effective by said release control means responsive to a disconnect signal to transmit over said trunk line a disconnect acknowledgment signal consisting of current impulses of two particular frequencies alternately, control means at said one end of said trunk line for terminating the transmission of said disconnect signal, means operated by disconnection of aid link from said trunk circuit for rendering "said control means operatively responsive to said disconnect acknowledgment signal, control means at said other end of said trunk line for terminating the transmission of a disconnect acknowledgment signal responsive to termination of a disconnect signal incoming over said'trunk line, andmeans at each end of ,said trunk line for marking said trunk line busy while a disconnect acknowledgment signal is being transmitted 'thereover.

6. In a telephone system, an interofilce trunk line, trunk and signaling circuits terminating one end of said trunk line, similar 'trunk'and signaling circuits terminating the other end of said trunk line, means ateach-end-of said trunk line -for seizing the trunk circuit thereat on an outrentof the second of said two frequencies for apredetermined interval of time and then i'n'recurring cycles current impulses of said two frequencies alternately for-a predetermined interval oftimefollowedby current of the second of said two Tfrequencies -'fora predetermined interval of time, release control means at each end of said trunk line responsive to-a disconnectsignal transmitted from the other end of said trunk line, means in the signaling circuit at each end ef 'sa-id trunk line rendered operativ by the assoiciated releasecont-rol means responsive to aidisconnect signal to transmit-over sai'd trunkulin'e a disc'onnect acknowledgment signal consisting of-=cur-rentimpulses- 0f two particular frequencies alternatelyyc'ontrol :means in the signaling circu it= at each end of said trunk line ior terminate :ing the transmission of said disconnect signal,

line, means at each end of said trunk :line for seizing the :trunk circuit thereat ion an ioutgoing calL'i means at each end of :said :trunk .line rerespons'ive to seizure of the trunk circuit thereat tor-conditioning the signaling circuit for the 7 transmission of -a disconnectsignal" responsiv to release or the' associated trunk circuitflsaidilast mentioned means-at :each end -being inoperative in case said trunk line i simultaneously seized *at both ends, means in the signaling circuit at it each end ofsaid "trunk line responsive to release of the trunk circuit thereat "for "transmitting :a "disconnect signal consisting not current :of iacfirst one of two particular frequencies :followed by current of the second of said two frequencies for 'a predetermined interval of time and then in recurringcycles current impulses of said two frequencies alternately for a predetermined interval of time followed by current of the second of said two frequencies for a predetermined interval of time, release control means at each end of said trunk line responsive to a disconnect signal transmitted from the other end of said trunk line, means in the signaling circuit at each end of said trunk line rendered operative by the associated release control means responsive to a disconnect signal to transmit over said trunk line a disconnect acknowledgment signal consisting of current impulses of two particular frequencies alternately, control means in the signaling circuit at each end of said trunk line for terminating the transmission of said disconnect signal, means in the signaling circuit at each end of said trunk line effective upon release of the trunk circuit thereat for rendering the last mentioned control means operatively responsive to a disconnect acknowledgement signal incoming over said trunk line, and control means in the signaling circuit at each end of said trunk line for terminating the transmission of a disconnect acknowledgement signal responsive to termination of a disconnect signal incoming over said trunk line.

8. In a telephone system, an interoflice trunk line, trunk and signaling circuits terminating one end of said trunk line, similar trunk and signaling circuits terminating the other end of said trunk line, means at each end of said trunk line for seizing the-trunk circuit thereat .on an ioutgoin'g call, meansat each end -'of said trunk line responsi-v'e to seizure-ofthe trunk circuit thereat for conditioning the signalingcircuit for the transmission of a-disconnect signal responsive to release of the associated trunk'circlntsaid last mentioned :means at each :end being inoperative incase saidtrunk line issimultaneously seized *at bothends, means in the signaling circuit at each end of said trunk'line responsive to release -of the trunk circuit thereat for transmitting a disconnect signal consisting or current of a first one of two particular frequencies followed by current of the second of said two frequencies for a predetermined interval of time and then in recurring cycles current impulses of said two frequencies alternately for a predetermined interval of time followed by current of the second of said two frequencies for a predetermined interval of time, release control means'at each end of said trunkline responsive to "a disconnect signal-transmitted from the other end of "said trunk line, means in the signaling circuit" at each end oi said trunk *line rendered-operative by the associated release control means responsive to a disconnect signal-to transmit' over said trunk lines disconnect acknowledgment signal consisting of current impulses of two particular =frequencies alternately, control means in the signaling-circuit at each end of said trunk line for terminating the transmission-0f said disconnect signal, means in the signaling :circuit at each end ofsaid :trunk line effective upon release of the trunk circuit-thereat for rendering the last" mentioned control means operatively responsive to a "disconnect acknowledgment signal incoming over said trunk line, control means in the signaling circuit at each end ofsaid -trunk line for terminating the transiiinef-trunkand signaling circuits terminating one end of said trunk ling-similar trunk an'd signaling circuits terminating the other end of said trunk line, means at each end of said trunk line for seizing the trunk circuit thereat on an outgoing call, means at each end of said trunk line responsive to seizure of the trunk circuit thereat for conditioning the signaling circuit for the transmission of a disconnect signal responsive to release of the associated trunk circuit, said last mentioned means at each end being inoperative in case said trunk line issimultaneously seized at both ends, means in the signaling circuit at each end of said trunk line responsive to release of the trunk circuit thereat for transmitting a disconnect signal consisting of current of a first one of two particular frequencies followed by current of the second of said two frequencies for a predetermined interval of time and then in recurring cycles current impulses of said two frequencies alternately for a predetermined interval of time followed by current of the second of said two frequencies for a predetermined interval of time, release control means at each end of said trunk line responsive to a disconnect signal transmitted from the other end of said trunk line, means in the signaling circuit at each end of said trunk line rendered operative by the associated release control means responsive to a disconnect signal to transmit over said trunk line a diSCOIl-y the associated release control means responsive to an incoming disconnect signal,- whereby -the transmission of a disconnect acknowledgment signal :from either end is prevented in case the trunk'circuitsrat both ends of saidtrunk 'lines re simultaneously s z d, o tro means 1 1117 signalingcircuitat each end of {said trunk line for terminating ;;the transmission of said disconnect signal, and means in'the signaling circuit at each end'of said trunk line efiective-upon release of: the trunk circuitthereatafor rendering the last mentioned control-means operatively responsive at each-end o sa d t n in being r ppnsi e tetha-t. par o an inco i c ne t i nal-wh ch cons stsu n impu ses f said .two irequencies; alternately fora predetermined interval of time, thereby topl terminate the'transmi ion oi -t e d sconn ct -s nal' the e rom: d .thus, enable the transmission j of a -.disconnect acknowledgment signaltherefrom responsive to the incoming disconnect signal. v

;'1O.,In a signaling systemiin which signals consist of alternatinglc rrent oi either; one of two signaling frequencies followedby current of the other of saidfrequencies a signal receiver comprising a firstrelay operatively responsive only to; current of one 1 of said; frequencies, a: second relay: and a third; relay eachoperatively responsive onlyto currentof the other of said f requencies said third relay beingnormally nonresponsive,.relay means responsive tothe operation of said first. relay for enabling the operation of said third relay, a normally inoperative signal r el ay,;relay means responsive to the operation of said second relay within a predetermined interval after the release ofsaid 'first relay for rendering said signalrelay operative, and circuit means includinga delay network connecting the operating winding of said signal relay to the contacts of said third relay, said delay network being effective totdelay for a predetermined interval of time the operative response of the signal relay to each operation of said third relay.

,11. In a signaling system, a first signal relay, responsive to direct current signals, a line, a signal transmitter connected to one end of said line and controlled by said first signal relay for transmitting over said line alternating current signals consisting of an impulse of current of either one of two particular frequencies followed by an impulse of current of the other of said frequencies, said transmittercomprising a source of current of each of said frequencies, and relay means for connecting said-sourcesto said line one after the other, said relay means including a control relay for transferring the line from one source to the .to adisconnect acknowledgment signal incoming over said trunk line, said. last'mentioned control said third relayn lrelay operatively responsivei only to current of one 10f :said frequencies, a second relay and a third relay each operatively responsive only to current of the other of'saidfrequenoies, said third relay being normally "non-responsive, relay means responsivetovthe operationof said first relay for enabling the operation of said third relay, a normally'ino'perative signal relay, relay means respon'sive to,the operation of said second relay with agpredetermined interval after the release ofisaid first relay :for rendering said si nal relay operative, andcircuit 'means'including a delay network connecting the operating winding of said signal relay. tothecontacts of said; third relay, said'delay network being'efiective to delay for a predetermined interval of time'the operative response of :thesignal relay to each operation of :ELLIOT M. STAPLES. 1:- EDMUND R. TAYLOR.

HENRY J. MICHAEL. 

